Field of the Invention
The invention relates to devices and methods for signal filtering and amplitude control. More particularly, it relates to ferrite resonator, such as yttrium-iron-garnet (YIG) resonator, based signal filtering and amplitude control methods.
Description of the Related Art
YIG-resonator based microwave filters, often formed from a single small sphere of Yttrium Iron Garnet material, are widely used in the microwave field. Such filters are described by the work of Moore, U.S. Pat. No. 3,740,675, Korber, U.S. Pat. No. 4,965,539, and others.
See also: Carter, Magnetically Tunable Microwave Filters Using Single-Crystal Yttrium-Iron-Garnet Resonators, ” in IEEE Transactions on Microwave Theory and Techniques, vol. MTT-9, 1961, pp. 252-260; J. D. Adam, L. Davis, G. Dionne, E. Schloemann and S. Stitzer, “Ferrite Devices and Materials,” in IEEE Transactions on Microwave Theory and Techniques, vol. MTT-50, 2002, pp. 721-737; and D. Harris, “A 4-40 GHz Wide Bandwidth, Magnetically Tuned Bandpass Filter;” in IEEE International Microwave Symposium Dig., 1990, pp. 1019-1022.
YIG resonator based filter devices have several advantages, specifically high operating frequency and wide tuning range, and are widely used in microwave test-and-measurement instruments such as signal generators and spectrum analyzers.
In addition, these instruments often also use one or more separate electromechanical or electronic attenuator devices for signal level control, as is depicted in FIG. 1. Such attenuators are typically used to adjust the output voltage of these microwave test-and-measurement instruments to a desired range. For example, an input microwave signal (10) spanning a plurality of microwave frequencies may be input into a YIG resonator based filter device (20), which will typically only pass a limited subset of the input frequencies. The output (30) from the YIG resonator based filter device is then adjusted to a desired output power range (often measured as a voltage range) by feeding the output into a separate attenuator device (40), thus producing an attenuated output (50) microwave signal covering the filtered range of frequencies, with a maximum power (and hence voltage) level controlled by the setting of separate attenuator device (40).
Electromechanical attenuator devices are often used for these applications (e.g. 40) because they can provide a very low insertion loss and a wide amplitude control range. However due to their mechanical operation principles, the switching speed and life time of such electromechanical attenuator devices can be limited. Alternatively, certain types of solid state attenuators can also be used, but these have other drawbacks, which will be discussed shortly.
A simplified block diagram of a conventional (prior art) YIG filter (100) is shown in FIG. 2.
A typical YIG filter often comprises a high-Q spherical YIG resonator element (102) (e.g. a YIG sphere), that is held in a magnetic field (see FIG. 3).
Typically a piece of wire (104), often called an input coupling loop, but here referred to as an input coupling wire, is partially wound around the YIG sphere (102) (often about half of the YIG sphere), and is positioned so that the coupling wire (104) can magnetically couple to the YIG sphere (resonator). Another piece of wire (106), often called an output coupling loop, but here referred to as an output coupling wire, is also partially wound around the YIG sphere, and also magnetically couples to the YIG sphere (resonator) (102). The input signal (10) is then magnetically coupled to the output signal (30) via the magnetic coupling between these two wires and the YIG sphere, and this coupling varies according to the resonant frequency of the YIG sphere (resonator) (102).
One or more YIG filters may be optionally combined, and configured to produce either bandpass filters or notch (e.g. bandreject) filters.
A simplified cross-sectional view (200) of an exemplary magnetic structure of a conventional (prior art) YIG filter is shown in FIG. 3. The magnetic field (202) is typically provided, at least in part, by an electromagnetic coil (204) (such as an electromagnetic tuning coil). This adjusts the strength of the magnetic field surrounding the YIG sphere, and hence the resonant frequency of YIG sphere, and hence can be used to tune the frequency of the YIG filter.
In some embodiments, the strength of the magnetic field can be further enhanced by use of a larger electromagnet and/or permanent magnet (not shown). The spherical YIG resonator (102) and the wires (not shown) are placed between the poles (206, 208) of this electromagnet. Frequency tuning can be provided by changing the magnetic field (202), with fine tuning often provided using a variable DC current flowing through the electromagnet tuning coil (204).
As discussed above, the frequency of a YIG resonator can be adjusted by varying the intensity of the YIG sphere's (YIG resonator's) magnetic field (202). This tuning is possible since the resonant frequency of the isotropic YIG resonator (102) in a uniform magnetic field is a nearly linear function of the magnetic field strength. The basic relationship between the YIG resonator resonant frequency ƒ and magnetic field strength H is given as follows:ƒ=γH, where: γ=2.8 MHz/Oe is the gyromagnetic ratio.
The gyromagnetic ratio is a physical constant, which is independent of the YIG resonator size.
Unfortunately, YIG resonator frequency is not perfectly stable. It is often temperature dependent, in part because the dimensions of the magnets used to create the magnetic field, and thus the magnetic field strength, can vary somewhat with temperature. To compensate, in prior art YIG resonator based filters, the YIG spheres are often heated above ambient temperature, and then temperature controlled using various temperature control methods.